Recycle Elution

The recycle elution method can be applied to mixtures of very similar compounds that cannot be fully separated by a single pass through the column. This method makes more effective use of a column. The effluent from the column is repeatedly re-passed through the same column. The number of cycles multiplies the total theoretical plate number of the column if the system is 

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The tert-huty hydroperoxide is then mixed with a catalyst solution to react with propylene. Some TBHP decomposes to TBA during this process step. The catalyst is typically an organometaHic that is soluble in the reaction mixture. The metal can be tungsten, vanadium, or molybdenum. Molybdenum complexes with naphthenates or carboxylates provide the best combination of selectivity and reactivity. Catalyst concentrations of 200—500 ppm in a solution of 55% TBHP and 45% TBA are typically used when water content is less than 0.5 wt %. The homogeneous metal catalyst must be removed from solution for disposal or recycle (137,157). Although heterogeneous catalysts can be employed, elution of some of the metal, particularly molybdenum, from the support surface occurs (158). References 159 and 160 discuss possible mechanisms for the catalytic epoxidation of olefins by hydroperoxides. 

PT catalysts are often difficult to separate from the product, while it is also desirable that the catalyst should be reusable or recyclable. Distillation and extraction are the most common separation processes. The main disadvantage of lipophilic quats is their tendency to remain in the organic phase and consequently contaminate the product. Therefore, extraction in water often is not satisfactory. Furthermore, products in the fine chemicals industry often have high boiling points and/or are heat sensitive, which makes separation of the catalyst by distillation impossible. Often the only means to remove the catalyst in these cases is to adsorb it using a high surface area sorbent such as silica, Florisil or active carbon (Sasson, 1997). After filtration, the catalyst can then be recovered by elution. 

Both types of units have generally been operated in trace mode that is, background or elutant electrolyte is fed to the unit along with the mixture to be separated. A desirable and possible means of operation for preparative applications is in bulk mode, in which one separated component follows the other without background electrolyte being present, except that other ions may be required to bracket the separated zones. Overlap regions between components should be recycled, and pure components collected as products. 

The chemical constituents of the backwash water (which may be acidified) and of the various eluants (recycling, new and special) are specific to the problem of displacing the captured ions from the resin and reactivating its sites. The final stage of IX, namely, the recovery in a solid form of the values from the eluting solution, is effected by chemical methods which yield the desired precipitate. This is settled out, filtered and dried for despatch. 

Figure 19.7. Cyclic batch elution chromatography obtaining high product purity and high throughput by using incomplete resolution (overlapping bands) and recycling the mixed fraction (mf) to the feedstock (a) Control of band separation and cut points determines fractional impurities t mij mr and Ami /mr2-<4l)> (b) Chromatogram for separation of pure ds- and trans- 1,3-pentadiene. Components 1, isoprene 2, trans- 1,3-pentadiene 3, cis-l,3-pentadiene 4, cyclo-pentadiene. Component 1 is eluted at almost the same time as component 4 of the...

Logan and Whitmore report that the dehydration rates for irradiated oligo U and poly U are functions of the chain length (Fig. 30) and pH. Furthermore, the dehydration rate for poly U which had been irradiated and dehydrated and then irradiated again was faster than that of the first dehydration. It was observed that the elution pattern on Sephadex of the recycled poly U had changed. 

Most process scale chromatographic separations are run under isocratic conditions, and therefore robust solvent recycling processes need to be designed. Figure 12.16 presents a simplified scheme coupling chromatography with eluent recycling [11]. The solvent is recovered from both evaporators and dryers while pure dry compounds are recovered. The recycled solvent is reused to elute the column and to dissolve the dry feed mixture. Only a small amount of fresh solvent is automatically added to the recycled solvent in order to adjust the eluent composition. 

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